I thought about this some more. If a family has two daughters, one of whom is named Mary, there's only a 50-50 chance that the parent will say "I have a daughter named Mary", so the families with two girls have the same chance of saying this as the families with one boy and one girl. Thus, it stays 1/3, I think. Bill C. -----Original Message----- From: math-fun-bounces+cordwell=sandia.gov@mailman.xmission.com [mailto:math-fun-bounces+cordwell=sandia.gov@mailman.xmission.com]On Behalf Of David Gale Sent: Wednesday, June 21, 2006 7:27 AM To: math-fun Subject: Re: [math-fun] Schroedinger's daughter The following clarified for me both the case of Mary and Bartholomew. There are two ways to pick a girl from a population of two child families. 1. Pick a family at random. If it contains a girl the odds are two to one that she has a brother. 2. Pick a girl at random. Then the odds are even that she has a brother, (because there are (roughly) the same number of girls with brothers as there are girls with sisters). Or is this is the classical rather than the quantum case. dg PS. Are there really twice as many boy-girl families as girl-girl or boy-boy? It shouldn't be too hard to check. Maybe certain couples are biologically more likely to produce one sex over the other. At 03:30 PM 6/20/2006, you wrote:

OK, I thought that I understood this, but here's a variation:

A friend writes a girl's name on a piece of paper and puts it (unopened) in your pocket. A random parent of two children comes up to you and says, "I have a daughter named Mary." At this point, the probability that Mary's sibling is a girl is 1/3 (I think). However, if you look at the piece of paper, and it says "Mary", the probability changes to 1/2 (approximately).

Is this correct?

Bill C. _______________________________________________ math-fun mailing list math-fun@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/math-fun

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